An organic electroluminescent device (hereinafter referred to as organic EL device) in the simplest structure is generally constituted of a light-emitting layer sandwiched between a pair of counter electrodes and functions by utilizing the following phenomenon. Upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer with emission of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state, is expected to enhance the luminous efficiency three to four times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer was investigated, but these derivatives merely produced luminance at an extremely low level. Thereafter, europium complexes were tried to utilize the excited triplet state, but failed to emit light at high efficiency. In recent years, as is mentioned in patent document 1, a large number of researches are conducted with the objective of enhancing the luminous efficiency and extending the life while giving priority to utilization of organic metal complexes such as iridium complexes.    Patent document 1: JP2003-515897 A    Patent document 2: JP2001-313178 A    Patent document 3: JP2004-273190 A    Patent document 4: JP2005-259412 A    Patent document 5: JP2006-199679 A    Patent document 6: JP2007-227658 A    Non-patent document 1: Applied Physics Letters, 2003, 83, 569-571    Non-patent document 2: Applied Physics Letters, 2003, 82, 2422-2424
In order to enhance the luminous efficiency, a host material to be used together with the dopant material becomes important. Of the host materials proposed thus far, a typical example is 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP), which is a carbazole compound presented in patent document 2. When used as a host material for green phosphorescent emitters, typically tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3), CBP exhibits relatively good luminous characteristics. On the other hand, CBP fails to perform at sufficiently high luminous efficiency when used as a host material for common blue phosphorescent emitters. This is because the lowest triplet energy level of CBP is lower than that of common blue phosphorescent emitters and the triplet energy of a blue phosphorescent emitter is transferred to CBP. That is to say, if a phosphorescent host material were designed to have triplet energy higher than that of a phosphorescent emitter, the triplet energy of the said phosphorescent emitter would be confined effectively and, as a result, the luminous efficiency would be enhanced. With the objective of improving the energy-confining effect, the structure of CBP is modified to increase the triplet energy thereby enhancing the luminous efficiency of iridium bis[2-(4,6-difluorophenyl)pyridinato-N,C2′]picolinate (hereinafter referred to as FIrpic) in non-patent document 1. Similarly, the luminous efficiency is enhanced by using 1,3-dicarbazolylbenzene (hereinafter referred to as mCP) as a host material in non-patent document 2. However, these host materials are not satisfactory in practical use, particularly from the viewpoint of durability.
Moreover, the host material needs to have well-balanced electrical charges (hole and electron) injection/transport characteristics in order to enhance the luminous efficiency. The electron transport property falls short of the hole transport property in the case of CBP and this disturbs the balance of electrical charges in the light-emitting layer and causes excess holes to flow out to the cathode side thereby reducing the probability of recombination of holes and electrons in the light-emitting layer and lowering the luminous efficiency. Furthermore, the recombination in this case occurs in a narrow limited region in the vicinity of the interface on the cathode side. Therefore, in the case where an electron-transporting material like Alq3 whose lowest triplet energy level is lower than that of Ir(ppy)3 is used, there may also arise the possibility that the luminous efficiency may become lower due to transfer of the triplet energy from the dopant to the electron-transporting material.
One of the means to prevent holes from flowing out to the electron-transporting layer is to provide a hole-blocking layer between the light-emitting layer and the electron-transporting layer. The hole-blocking layer accumulates holes efficiently in the light-emitting layer and contributes to improvement of the probability of recombination of holes and electrons in the light-emitting layer and enhancement of the luminous efficiency (patent document 2). The hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato)aluminum (hereinafter referred to as BAlq). These materials can prevent holes from flowing out of the light-emitting layer to the electron-transporting layer; however, the lowest triplet energy level of both of them is low compared with that of a phosphorescent dopant such as Ir(ppy)3 and sufficient luminous efficiency cannot be obtained.
Moreover, BCP tends to crystallize even at room temperature and lacks reliability as a hole-blocking material and the life of the device is extremely short. Although BAlq is reported to have a Tg of approximately 100° C. and provide the device with relatively good life, its hole-blocking ability is not enough. In addition, there arises a problem that an increment of one layer complicates the structure of the device and increases the production cost.
As described above, in order for an organic EL device to perform at high luminous efficiency, a host material is required to have high triplet energy and to be balanced in the electrical charges (hole and electron) injection/transport characteristics. Furthermore, the host material is desirably a compound endowed with electrochemical stability, high heat resistance, and excellent stability in the amorphous state. However, no compound capable of satisfying these properties on a practical level is known at the present time
Some of the organic EL devices developed so far by making use of phosphorescent molecules use materials containing a carbazole ring (carbazole skeleton) as host materials in the light-emitting layer. In recent years, several materials containing a carbazole ring have been proposed for the purpose of increasing the triplet energy.
A biphenyl derivative containing carbazole rings shown below is proposed as a host material in patent document 3.

In the aforementioned compound, the carbazole rings are linked to the aromatic ring of biphenyl skeleton and this configuration causes the electron transport property to fall short of the hole transport property in the molecule. Hence, the electrical charges are balanced poorly and the durability is insufficient. Furthermore, when attention is paid to two benzene rings in the aforementioned biphenyl skeleton, each of them is linked to the carbazole ring at the ortho position and this configuration develops a strain in the molecule and causes a loss of stability thereby emphasizing all the more the aforementioned problem.
The following compound and others are shown as luminous materials in patent document 4.

However, in the aforementioned compound, the benzene ring at the center is trivalent and linked to three 5′-carbazolylbiphenyl groups like a starburst structure. Hence, it is likely that the triplet energy becomes lower and sufficient luminous efficiency cannot be obtained.
Further, the following compounds are proposed as luminous materials in patent document 5.

However, in the aforementioned compounds, one benzene ring is substituted with two carbazolyl groups and, when the compounds are subjected to electrical oxidation/reduction, it is conceivable that localization of electrical charges between the carbazole ring responsible for the hole transport property and the benzene ring responsible for the electron transport property becomes extensive and the result is a lack of electrical stability.
The following compound is shown as a luminous material in patent document 6.

However, the aforementioned compound possesses two terminal carbazolylphenyl groups respectively substituted at the ortho position. Hence, the compound has high triplet energy and the luminous efficiency is enhanced. However, a large strain develops in the molecule when all the benzene rings are linked to one another at the ortho position as in the aforementioned compound; in such a case, charge delocalization becomes difficult to occur between the aromatic rings when the compound is subjected to electrical oxidation/reduction, an excessive load is imposed on the molecule, and the durability becomes insufficient.